The 8-string “Brahms guitar” was the invention of the guitarist Paul Galbraith and the luthier David Rubio in the mid 1990s. You can read something about the original development of the design on David Rubio’s website: http://www.rubioviolins.com and Paul Galbraith’s: http://www.paul-galbraith.com/engl/8string.htm , so I won’t repeat that part of the history of the instrument here.
I continued the development of the Brahms guitar after David Rubio’s death in 2000, building my first one in 2001. Since then I have built around 40 of them and have developed and improved the design considerably. The main areas of the design which needed development are the structural strength and stability of the soundboard, and the tonal balance across the strings.
The tonal development has mostly been centred on improving the tone of the high A string, and making it balance with the other trebles, while also keeping a deep and full tone for the low end of the range. I think that my current design is very successful in balancing the tone and volume across the full range of the 8-string.
The structural challenge has been to make a soundboard which responds and sounds like a classical guitar, but which is capable of supporting the extra string tension of an 8-string guitar (which is closer to the tension of a steel-string guitar). Over the last 15 years I have tried various different solutions to that problem – different bridge designs, tailpieces and soundboard/bracing designs, and have now settled on a design which is both stable and responsive.